1. Field of the Invention
The present invention relates to a light-emitting element, a lighting device, a light-emitting device, and an electronic device. In particular, the present invention relates to a light-emitting element, a lighting device, a light-emitting device, and an electronic device utilizing electroluminescence (EL).
2. Description of the Related Art
Research and development have been extensively conducted on light-emitting elements utilizing EL. In a basic structure of the light-emitting element, a layer containing a light-emitting organic compound (hereinafter also referred to as an EL layer) is sandwiched between a pair of electrodes. The light-emitting element utilizing EL has attracted attention as a next-generation flat panel display element owing to characteristics such as feasibility of being thinner and lighter, high-speed response to input signals, and capability of direct current low voltage driving. In addition, a display using the light-emitting element has a feature that it is excellent in contrast and image quality, and has a wide viewing angle. Further, since the light-emitting element is a plane light source, application of the light-emitting element as a light source such as a backlight of a liquid crystal display and an illumination device is proposed.
In the case of a light-emitting element in which a layer containing an organic compound used as a light-emitting substance is provided between a pair of electrodes, by applying a voltage to the element, electrons from a cathode and holes from an anode are injected into the layer containing the organic compound and thus a current flows. The injected electrons and holes then lead the organic compound to its excited state, so that light emission is obtained from the excited organic compound.
As the excited state caused by an organic compound, there are a singlet excited state (S*) and a triplet excited state (T*). Light emission from a singlet excited state is referred to as fluorescence and light emission from a triplet excited state is referred to as phosphorescence. Here, in a compound that emits fluorescence (hereinafter also referred to as a fluorescent compound), in general, phosphorescence is not observed at room temperature, and only fluorescence is observed. Accordingly, the internal quantum efficiency (the ratio of generated photons to injected carriers) of a light-emitting element including the fluorescent compound is assumed to have a theoretical limit of 25% based on the ratio of the singlet excited state to the triplet excited state.
Meanwhile, when a compound that emits phosphorescence (hereinafter also referred to as a phosphorescent compound) is used, the internal quantum efficiency can be theoretically increased to 100%. That is, higher emission efficiency can be obtained than using a fluorescent compound. For these reasons, a light-emitting element including a phosphorescent compound has been actively developed in recent years in order to obtain a light-emitting element with high emission efficiency.
As the phosphorescent compound, an organometallic complex that has iridium or the like as a central metal has particularly attracted attention because of its high phosphorescence quantum yield; for example, an organometallic complex that has iridium as a central metal is disclosed as a phosphorescent material in Patent Document 1.